Joint Completion Report on IDNP Result # 3 ”Computer Modeling i17 Irrigation and Drainage”

9. SALT AND WATER BALANCE MODELLING: TUNGABHADRA ’ IRRIGATION PROJECT (UASD)

9.1 Introduction

The Tungabhadra irrigation project is a major inter-state irrigation project of peninsular India. It was commissioned during 1953 with an irrigation potential of 3.63 lakh hectares in Karnataka and 1.6 lakh hectares in Andhra Pradesh. The project was designed for protective irrigation and almost three-fourth of the command area was earmarked for light irrigated crops.

Prior to 1990, cotton was the major crop and irrigation was mainly protective after cessation of monsoon rains. However, in recent years, frequent failure of cotton due to pests and diseases and secondary salinisation of the area, has resulted in paddy overtaking cotton. Rice-rice cropping sequence has become the mainstay of the Tungabhadra irrigation command. According to the CADA report only 8 per cent (29,032 ha) was localised for rice cultivation. The current estimates are that the rice is cultivated over 80,000 ha in left bank canal alone with progressive increase each year. This has created a serious imbalance in the per capita availability of water in the irrigation command. Use of water at the head end command is substantially high due to rice cultivation, while large areas at the tail end suffer either due to little or no water.

After the introduction of irrigation, the ill-effects of waterlogging and salinity are overwhelming in the command area due to many reasons. The extent of problem, which was just under 20,200 ha during 1979-80 has risen to over 80,000 ha during 1996-97. It seems that since 1979-80, the area under waterlogging and soil salinisation is increasing at the rate of 3,000 ha per annum. It has been estimated that there is a progressive salinisation of the command over the years. Soil salinity has increased from 0.6 to 5.5 dS/m in l m depth of soil in the last 35 years. Canal seepage is believed to be one of the major causes of water table build-up and soil salinisation. Farmers at the tail end of the irrigation command have resorted to use drain/nala/underground poor quality waters and it has added a new dimension to the problem.

Inequitable distribution and loss of water has resulted in non-availability of irrigation water to 70,000 to 80,000 ha in the tail-end command and further a loss of 80,000 ha due to waterlogging, salinity and alkalinity. The excessive irrigation in upper reaches has caused a significant decline in the potential profitability of the command area. According to a conservative estimate, loss in crop production is about Rs. 300-500 millions per year due to non-availability of water while, an additional Rs. 250-300 millions per year due to waterlogging and salinity.

SALTMOD is a computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the water table and the drain discharge in irrigated agricultural lands. It requires information on different (geo)hydrologic conditions, varying water management options, including the use of groundwater for irrigation and crop rotation schedules. This model is used in the present study to analyse the change in salt and water balances after the introduction of subsurface drainage and to make long-term predictions of soil salinity and depth to water table in the pilot area.

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Joint Completion Report on IDNP Result # 3 "Computer Modeling in Irrigation and Drainage"

9.2 Study Site

The topography of the Left Bank Command area is rather undulating and only occasionally gives the impression of a flat plain. The canal is a contour canal and forms the boundary of the command areas of two adjacent distributaries. Field experiments on an interceptor drainage system were conducted during 1998-2001 at distributary D-36/1, covering an area of 62 ha. This distributary runs on a well-defined ridge perpendicular to the main canal. A three-tier interceptor drain of 10 cm diameter was laid at a depth of 0.75 m from the surface to intercept the incoming seepage flows from canal and prevent waterlogging and soil salinisation in low lying areas. The drains were laid at a spacing of 150 m running parallel to D-36/1 near Sindhanur during 1998. The drain placement is about 100 m away from the natural drain (Vade halla) and 1500 m from the canal. The interceptor drains are laid with corrugated and perforated PVC pipes with filters. '

Performance of interceptor drainage system was evaluated in terms of changes in soil salinity, depth to water table, crop yield, cropping intensity, etc. The study was initiated during kharif 1998 and continued up to kharif 2001. Changes in soil salinity due to interceptor drainage system were evaluated by collecting soil samples at 12 grids points. The performance of interceptor drainage system was also monitored in terms of changes in depth to water table. Water table depth was recorded in each crop season after the harvest of the crop. To assess the amount of salts removed from the study area, drain discharge was measured periodically and analysed for its quality. To study the individual performance of the three parallel interceptor drains, individual drain discharge and its salinity was measured separately during Aug. 2000 to May 2001. These three drains removed almost same quantity of drainage water and their salt concentration is also more or less similar. It indicates that the drainage water removed through the drains is mostly from the paddy area than intercepted seepage from the canal.

9.3 Model Calibration

Üsing the data collected at the pilot study area and by estimating some of the parameters, the input file for SALTMOD is prepared and presented in Tables 24 and 25. Values of few of the parameters are assumed logic'ally. For example, the leaching efficiency of the root zone and transition zone were calibrated using the field data and were taken as 0.70, while the leaching efficiency of the aquifer was taken as 1.00.

9.4 Scenario Building

9.4.1 Prediction of Future Soil Salinity and Depth to Water Table

SALTMOD was run for 20 years period for the prediction of root zone salinity and water table depth. The salinity of soil moisture in the root zone (Cr4) is observed to reduce considerably due to the influence of drainage systems in the pilot area (Fig. 22). This figure shows that, for any year, the Cr4 values in the first cropping season are observed to reduce due to leaching by the high amounts of irrigation water and rainfall. After this, the soil salinity is further reduced in the second cropping .season due td leaching by the huge quantities of irrigation water applied for the paddy crop.

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Joint Completion Report on IDNP Result # 3 "Coniputer Modeling in Irrigation and Drainage"

Table 24. Season-wise input parameters for use with SALTMOD

SI. Parameters No.

Season 1 Season 2 Season 3 Season 4

1

2

3

4

5

6

7

8

9

10

11

12

13

Duration 1 August to 31 December

(5 months)

Crops grown Paddy

Source of water Rainfall and

Fraction of area occupied by irrigated crops

Fraction of area with unirrigated crops

Fallow 0.05

Rainfall (m) 0.55

Canal water used for irrigation (m) 0.55

Groundwater used for irrigation Nil

Potential evapotranspiration of crops (m) 0.72

o. 1

Irrigation water

0.95 * . .

Percolation from canal system (m)

Outgoing surface runoff (m) 0.122

Out going groundwater flow through aquifer (m) 0.10

1 January to 31 January (1 months)

Fallow

Canal water

1 .o 0.0

0.0

Nil

0.075

0.0

0.0

0.09

1 February to 31 May

(4 months)

Paddy

Irrigation water

0.95

0.05

0.05

1 .O5

Nil

0.73

o. 1

0.240

0.10

1 June to 31 July

(2 months)

Fallow Nil

1 .o 0.0

Nil

0.34

0.0

0.0

0.09

Table 25. Other input parameters for use with SALTMOD

SI. No. Parameter Value

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Storage efficiency

Depth of root zone

Depth of tiansition zone

Depth of aquifer

Total pore space of root zone

Total pore space of transition zone

Total pore space of aquifer

Effective porosity of root zone

Effective porosity of transition zone

Effective porosity of aquifer

Initial salt content of the soil moisture (dS/m) at field saturation in root zone/transition zonelaquifer

Salt concentration of canal water (dS/m)

Initial depth to water table from ground surface

Critical depth of water table for capillary rise

Leaching efficiency of root zone/transition zone/aQuifer

0.80

0.50 '

1.50

25.0

0.40

0.40

0.40

0.20

0.20

0.30

8.5/10.0/10.0

0.30

' - 0.45

1 .o0

0.70/0:70/1 . O 0

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Joint Completion Report on IDNP Result # 3 “Computer Modeling in lrrigation and Drainage”

Decreasing trend in root zone soil salinity’is observed in the first three years. During this period, the salinity decreased from its initial value of 8.5 dS/m to 1.5 dS/m during the kharif, while in rabi it was 2.3 dS/m. Thereafter, the root zone salinity is practically constant and is maintained at the same level. By the end of 20 years, the predicted root zone salinity is 1.5 dS/m and 2.2 dS/m during kharif and rabi, respectively. If one compares the actual values (three years) with the simulated values, more or less similar trend is observed in both the cases, while a slight variation in the absolute values is noticed in both the seasons. The salt concentration in the transition zone (above and below the drain) is also reduced considerably due to influence of subsurface drainage system. Decreasing trend in the salinity is observed throughout the predicted period. By the end of 20 years, the predicted salinity above the drain is 1.9 dS/m and 1.8 dS/m during kharif and rabi, respectively. The predicted salinity below the drain is 1.8 dS/m and 1.5 dS/m during kharif and rabi, respectively. Gradual reduction in soil water salinity in the aquifer (Cqf) is also observed over a period of time. By the end of 20 years, the salinity decreased from its initial value of 10.0 dS/m to 5.5 dS/m.

The simulated depths to water table indicated that the water table remains shallow ( 0.65 to 0.7 m) in both the seasons even after the provision of subsurface drainage system (Fig. 23). This is mainly due to shallow drain depth (0.75 m). By comparing the actual values (three years) with the simulated values, a deeper water table depth is noticed under the actual field condition than simulated during the rabi period. This is mainly due to canal closure and peak summer period. During kharif, a slight variation in the absolute values is noticed and this variation is only to the extent of 5 to 10 cm. About 25 and 15 cm of water percolates during kharif (rainy) and rabi (summer) seasons respectively, due to high rainfall received and excessive irrigation water applied to the crop. No capillary-rise is observed in both the seasons. The achieved irrigation efficiency and sufficiency is about 65 and 98 per cent, respectively.

9.4.2 Effect of Controlled Drainage on Root Zone Salinity and Water Table

In the pilot study area, farmers started blocking the drainage system after two years of installation. The effect of controlled drainage on root zone salinity and changes in water table depth is simulated for three different conditions i.e. blocking the drains during both rabi and kharif crop seasons, blocking during rabi only and blocking during critical crop stages in both the seasons. In all the three cases, 30 cm depth of irrigation water is applied in the second season to meet the leaching requirement of the soils. Since, there is no supply of canal irrigation in the fourth season, no irrigation can be provided during this season to meet the leaching requirement. The time weighted leaching efficiency (Fh) is calculated for all the three different controlled drainage conditions and used for prediction. The simulated values through the SALTMOD after season four were used for prediction. Fig. 24 and Fig. 25 show that in case of blocking during both rabi and kharif crop seasons, root zone salinity will increase considerably over a period of time even after applying 30 cm irrigation water in the second season for leaching. The salinity increased from 2.2 dS/m to 4.1 and 2.7 dS/m in the first year itself during rabi and kharif, respectively. Salinity increased further to 5.6 dS/m and 4.5 dS/m in the second year during rabi and kharif, respectively. By the end of 20 years, the root zone salinity is increased to about 12 dS/m in both the seasons. It indicates that complete blocking of the drainage system during cropping period had adverse effect on soil salinity.

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Joint Completion Report on IDNP Result # 3 "Computer Modeling i17 Irrigation arid Drainage"

'" I Predicted - Kharif -+-Rabi

o ' O 2 4 6 8 10 12 14 16 18 20

Year

Figure 22. Comparision of actual and predicted root zone salinity

Year

O 5 10 15 20 O

Predicted - Kharif -Rabi Actual - - + - - Kharif -a- Rabi

,- ,'

02

- E

0 4

.-4 . . . - - - - - - - - - - - - _ - _ - - - - - - '. 0 8

1

Figure 23. Comparision of actual and predicted depth to water table

In case of blocking during rabi season and blocking during critical crop stages, a marginal increase in salinity is observed in both the cases and in both the seasons. By the end of 20 years, the root zone salinity is increased marginally from 2.2 dS/m to 3.1 and 3.6 dS/m during rabi in case of blocking during rabi season and blocking during critical crop stages, respectively. During kharif, root zone salinity is increased marginally from 2.2 dS/m to 2.3 and 2.8 dS/m in case of blocking during rabi season and blocking during critical crop stages, respectively. This shows blocking the drainage system during rabi and during critical crop stages in both the seasons had less effect on soil salinity and the root zone salinity is maintained within the safe limit (c 4 dS/m).

Effect of controlled drainage on changes in water table depth for different blocking conditions is shown in Fig. 26 and Fig. 27. During rabi, water table is raised from itsinitial level of 0.68 m below

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1 Joint Completion Report on IDNP Result # 3 "Computer Modeling in lrrigation and Drainuge"

ground surface to 0.34 m in case of blocking during both rabi and kharif crop season. Marginal rise in water table is also observed in case of blocking during rabi only ( 0.50 m) and blocking during critical crop stages in both the seasons (0.61 m). During kharif, water table rises sharply from its initial level of 0.68 m below ground surface to 0.0 m (i.e. on the ground surfaces) in case of blocking during both rabi and kharif crop seasons. Marginal rise in water table is also observed in case of blocking during rabi only (0.63 m) and blocking during critical crop stages in both the seasons (0.56 m). This shows that blocking of the drainage system in both the seasons should be avoided. Even.though a marginal rise in water table is observed in case of blocking during rabi only and blocking during critical crop stages in both the seasons, the water table is deeper (< 0.5m) and should have no adverse effect on crop.

-80th the crop period -Rabi crop -Critical period

14 I I

- 1 _ _ _ _ _ _ - - - - - -

O 5 10 15 20

Year

Figure 24. Effect of controlled drainage on soil salinity (Rabi)

-80th the crop period -Rabi crop -Critical period

14 1 12

1 o , 5 10 15 20

Year

Figure 25. Effect of controlled drainage on soil salinity (Kharif)

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Joint Completion Report on IDNP Result # 3 “Computer Modeling in Irrigation and Drainage”

Year

O 5 10 15 20 O I

n 0.2 1 E ’

+ 2 0.6 E Y

-Both the crop period -Rabi crop +Critical period

Figure 26. Effect of controlled drainage on water table depth (Rabi)

O

Year

5 10 - 15 20

, I

-.c Both the crop period - Rabi crop -t Critical period

1 I Figure 27. Effect of controlled drainage on water table depth (Kharif)

9.4.3 Effect of Varying Drain Spacing on Root Zone Salinity

The drain spacing is represented by a parameter, QH1 in the input file. The higher the QH1 value, the spacing between the drains will be the narrower. The QH1 value under the present drain spacing in the pilot area is 0.01. In order to simulate the effect of drain spacing on root zone salinity, the QH1 values were varied in the range of 60% (QH1= 0.006), 80% (QHl= O.OOS), 120% (QH1=0.012) and 140% (QH1=0.014) of the present value. It was observed that the results of all the simulations almost overlaped and eventually lead to the same soil salinity as in the present system. Since the spacing of the drains is already 150 m against a recommended spacing of 50 m or so, such results seem reasonable. More over, minor numerical diffrences in the first two years are of little consequence in a system with a long life of 30 years or more. It seems that land reclamation is possible even with higher spacing of 150m in this area. Studies at several other locations in India have also confirmed that larger lateral spacing than the designed spacing has

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Joint Completion Report on IDNP Result ## 3 "Computer Modeling in Irrigation and Drainage"

I

helped to reclaim saline lands. Within 2-3 years, crops yield in large drain spacing become identical with the system laid with designed drain spacing.

9.4.4 Effect of Changes in Irrigation Water Applied on Root Zone Salinity

the model was run with water application of 60, 80, 120 and 140% of the existing level. As the

i i i,

f In order to simulate the effect of changes in irrigation water applied (IWS) on root zone salinity,

amount of irrigation water applied increased, the root zone salinity reduced faster because of more leaching of salts from root zone into deeper layers (Fig. 28). However, by applying 60 and 80% of present amount of irrigation water, the root zone salinity during first year is more than 5 dS/m and from the third year onwards the salinity is less than 4 dS/m. However, lesser irrigation sufficiency, i.e., 72 and 88 per cent is achieved in case of 60 and 80 O/O of present amount of irrigation water, respectively. This may affect the crop growth and result in lower crop yields. Application of more amount of irrigation water resulted in lesser irrigation efficiency (about 50 Yo). In the present study existing level of irrigation water applied had better irrigation efficiency (65 Yo) and sufficiency (100 "/o).

10

-60% -80%

o ' .O 2 4 6 0 10 12 14 16 10 20

Period (Years)

Figure 28. Variation in root zone salinity for different irrigation levels

9.5 Conclusions and Recommendations

On the basis of preliminary calibration and application of SALTMOD for salt and water balance modeling of the Tungabhadra project pilot area, the following conclusions are drawn:

SALTMOD is a very useful model for prediction of salinity of soil moisture, groundwater and drainage water, the depth of the water table and the drain discharge in irrigated agricultural land under varying water management options. Its simplicity of operation and minimum requirement of field data promotes its use by field engineers to obtain results that could be useful for planners. '

I

i

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Joint Completion Report on IDNP Result ## 3 “Computer Modeling in Irrigation and Drainage”

m e model predicts that due to the existence of drainage system, the root zone soil water salinity will be reduced to about 2.5 dS/m (from an initial value of 8.5 dS/m) within two years.

The model shows that controlled drainage during rabi only and during the critical crop stages in both the seasons had little effect on the build-up in root zone soil salinity; the root zone soil salinity is maintained within the safer limit (< 4 dS/m). But, complete blocking of the drainage system during both the cropping seasons had adverse effect and lead to build up of soil salinity soil salinity.

The present spacing of the drainage system seems reasonable to control water table and soil desalinization.

By reducing the amount of irrigation water by 20% of the present level, the root zone salinity can be brought down to 5 and 4 dS/m by the end of the second and third year, respectively. This will result, however, in a lesser irrigation sufficiency which may affect the crop growth and consequently lower crop yields, whereas the application of more irrigation water than presently used will result in a lesser irrigation efficiency (about 50 YO).

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Joint Completion Report on IDNP Result # 3 “Computer Modeling in Irrigation and Drainage”

REFERENCES

Dhaliwal, S. S., 1992. Effects of brackish water irrigation and straw mulching on salt accumulation and yield of summer moong (Vigna radiata L. Wilczek). Unpublished M. Sc. Thesis. Submitted to Department of Soil Science, Punjab Agricultural University, Ludhiana. India. 105 pp.

Jurriëns, M., Zerihun, Boonstra, J. and Feyen, J., 2001. SURDEV: Surface Irrigation Software. ILRI publication 59, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands, 194 pp.

Kroes, J.G., Dam, J.C. van, Huygen, J. and Vervoort, R.W., 1999. User’s Guide of SWAP Version 2.0. Technical Document 48, DLO-Winand Staring Centre, Wageningen, The Netherlands, 40 pp.

Oosterbaan, R.J., 2000. SALTMOD, Description of Principles, User Manual and Examples of Application. ILRI special report, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands, 80 pp.

Poonia, S. R., Singh, M., and Pal, R.,1990. Predicting sodification of soil upon irrigation with high residual sodium carbonate water in the presence and absence of gypsum. J. Indian Soc. Soil Sci. 38,713-718.

Simunek, J., Suarez, D.L., and Sejna, M., 1996. The UNSATCHEM Software Package for Simulating the One-dimensional Variably Saturated Water Flow, Heat Transport, Carbon Dioxide Production and Transport with Major Ion Equilibrium and Kinetic Chemistry. Version 2.0. Research Report 141, U.S. Salinity Laboratory, California.

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Joint Completion Report on IDNP Result # 3 "Computer Modeling in lrrigation and Drainage"

SYMBOLS USED

A D Alternate application of canal and drainage water

AG

cw Canal water

Cbot

Alternate application of canal and groundwater

Salt concentration of soil water at bottom of the profile (mg/ cm2)

Salt concentration of the soil moisture in the aquifer, when saturated, at the end of the season (EC in dS/m)

Salt concentration of the soil moisture in the root zone, when saturated, at the end of the season (EC in dS/m)

Salt concentration of the soil moisture in the transition zone aquifer above drain level, when saturated, at the end of the season (EC in dS/m)

Salt concentration of the soil moisture in the transition zone aquifer below drain level, when saturated, at the end of the season (EC in dS/m)

Seasonal average salt concentration of the soil moisture in the transition zone, when saturated, at the end of the season (EC in dS/m)

Cqf

Cr4

Cxa

Cxb

Cxf -

Dd

Ds

DW Drainage water

Dw

Depth of subsurface drains (m)

Salt storage in the root zone (mg/cm2)

Seasonal average depth of the water table below the soil surface (m)

Eact Actual evaporation (cm)

E=. Electrical conductivity of soil saturation extract (dS/m)

ET Evapotranspiration

Flr

Gd

Leaching efficiency of the root zone (-)

Total amount of subsurface drainage water (m3/season per m2 total area)

Horizontally incoming groundwater flow through the aquifer i (m3/season per m2 total area)

Gi

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Joint Completion Report on IDNP Result ## 3 "Cohputer Modeling in Irrigntion nnd Drninage"

Gn Excess of the horizontally outgoing over horizontally incoming groundwater flow through the aquifer (m3/season per m2 total area)

Horizontally outgoing groundwater flow through the aquifer (m3/season per m2 total area)

GO

G w Groundwater

IMD

P

Qbot

QH1

RSC

SAR

Tact a

DW

DS

Indian Meteorological Department

Irrigation water applied (cm)

Irrigation Water Applied during a particular season

Crop coefficient

Shape factor, Mualem-Van Genuchten functions (- )

Saturated hydraulic conductivity (cmd-')

Shape factor, Mualem-Van Genuchten functions (- )

Precipitation (cm)

Soil water flux at bottom of the profile (cm)

Ratio of drain discharge and height of the water table above drain level (m/day per m)

Residual sodium carbonate

Sodium adorption ratio

Actual transpiration (cm)

Shape factor, Mualem-Van Genuchten functions (cm-l)

Change in water storage (cm)

Change in salt storage (mg/cm2)

--

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